Signs of what could be flowing water have been found on Mars, Nasa has revealed, opening up speculation over whether life exists on the red planet.
In a statement released on Thursday, the space agency said dark, finger-like features appear and extend down some Martian slopes during late spring through to summer. Then they fade in winter, and return during the next spring.
"Nasa's Mars Exploration Program keeps bringing us closer to determining whether the Red Planet could harbor life in some form,” Administrator Charles Bolden said in a statement, “and it reaffirms Mars as an important future destination for human exploration."
The observations were made from striking images released by the Mars Reconnaissance Orbiter, which was launched in 2006.
Alfred McEwen of the University of Arizona, Tucson, who examined the orbiter's images, said the flow of briny water was the best possible explanation of what the dark tendrils could be.
His work on the seasonal flows was published in Thursday's edition of the journal Science.
Lewis Dartnell, an astrobiologist at University College London, told the BBC the findings could indicate that "hardy microbes" able to survive short periods were living on the planet.
"Liquid water is absolutely essential for life, and we've found life on Earth in pretty much every moist niche," he said.
The latest findings are said to be the strongest indicators so far that there could be water on Mars. Scientists have previously pointed to the possibility - in 2008 what appeared to be potential droplets of brine appeared on the Phoenix Mars Lander probe.
And in 2006 Nasa photographs revealed deposits in two gullies that could have been made by sediment carried by water in the recent past.